fix(managed-database skill): use orchestrator-platform run_migration

[rishikesh-major]

Summary

• update only the managed-database skill to replace four stale resource migration tool references with mcp__orchestrator-platform__run_migration
• document arguments as required migration and description, with applicationId only in org-scoped sessions
• keep the PostgreSQL resource skill unchanged from main

Checks

• git diff --check
• prospective diff against origin/main: one file, four line replacements

[rishikesh-major]

Superseded by #149: #149